Since the introduction of the legal requirement to wear the safety belt in 1977, as well as the introduction of the safety belt in the 1970s and of the airbag in the 1980s, the number of fatalities has been significantly reduced, from 21,000 per year to 3,500 per year. In addition to other restraint systems, the safety belt is responsible for almost 75 to 80% of the restraining action for the vehicle occupants in the case of an accident. A further development in this area was the introduction of systems having irreversible safety belt tensioners in the mid-1980s. In recent years, work has been done on systems that are designed to prevent accidents and to reduce the severity of accidents. In this connection, systems have been developed that range from interior compartment sensing to so-called pre-crash recognition systems.
In order to couple the occupants as early as possible to the vehicle deceleration in the case of an accident, for example excess belt looseness, caused for example by thick clothing or considerations of comfort, can be remedied by pyrotechnic tensioning after the beginning of the collision. Recently, reversible mechanical actuators have been used as belt tensioners, which can be activated in a critical driving situation or through environmental sensing, and which can very effectively remedy excess belt looseness, because they come into play before the collision. If a collision nonetheless takes place, then for example the pyrotechnic belt tensioning can in addition be activated. In this way, a still earlier coupling of the occupants can be ensured.
The targeted coupling of the passengers to a vehicle deceleration can cause very strong loads on the passengers, predominantly in the head and thorax regions. These loading peaks can be reduced for example by belt force limiters. The design here is based on the release of the safety belt starting from a specified safety belt force, for example starting from a force level in the range of from 2 to 3 kN. In general, the limitation of force can take place through mechanical energy conversion designs, for example destruction, for example via tear seams on the safety belt or tear plates, through deformation, for example through torsion bars in the belt roller, in some cases also having a multi-stage realization, or through friction, for example through multiple disk brakes. The intention is to ensure that, given a belt force that remains constant, the energy consumption takes place through increasing forward displacement of the occupants; i.e., a front airbag takes over the occupants from the belt system beginning from a particular point in time, for example after 40 to 60 ms. During the collision, the kinetic energy of the occupants should be distributed as advantageously as possible among the components of the occupant protection system and the interior compartment. Systems in use today reduce the belt force for example via a mechanical system, based on a coupled torsion rod, or via a wedge brake.